im stringing a racquet for a friend and my klippermate stringing guide says i'll need 21 feet of string for the mains. my question is does that 21' include extra lenght to tie the knots at the end? in other words, if i have exactly 21' feet of string for the mains, will i be able to string the mains or do i need extra length to the tie the knots?

BUT, in my experience, add a little more (say 8 inches) when you string with a poly. I've been caught out a couple of times measuring exactly the amount of string as specified and then being too short with poly. And it's not just the Klippermate pattern manual either - the official Prince stringing guide for the O3 Red mains gives a length way too short for syn gut, let alone poly.